Injection molding · Process validation
Injection Molding Cavity Weight Balance Calculator
Compare controlled short-shot weights across a multi-cavity mold without declaring an acceptance limit.
Formula and method
Imbalance (%) = (maximum cavity weight − minimum cavity weight) / mean cavity weight × 100. Each cavity deviation = (weight − mean) / mean × 100.
Inputs and units
Weigh cavity-separated parts on the same calibrated scale after a consistent conditioning interval. Use several repeated shots when making decisions.
Worked example
Four short-shot weights of 10.1, 9.9, 10.0 and 10.0 g have a 10.0 g mean and 2% max-to-min imbalance.
When to use the study
Use on cavity-numbered short shots before packing masks the original fill distribution.
How to interpret the result
A larger spread identifies a cavity-to-cavity difference for investigation. Review repeatability, dimensions, appearance, runner/gate condition and cavity position together.
Assumptions and limitations
No universal percentage certifies a balanced mold. Family molds, intentional geometry differences and measurement resolution change interpretation.
Validation guidance
Preserve machine, mold, material lot, melt/mold temperature, cycle state, units, instrument and sample order with the result. Repeat after any material, machine or tooling change.
FAQ
Does a low weight spread prove every cavity is equivalent?
No. Confirm repeated shots, critical dimensions, appearance and cavity-specific requirements.
Related workflow
Process Validation & Stability hub · Scientific molding validation workflow · Weight measurement guide · Gate seal and pressure-drop setup
Sources
PlasticsToday: Scientific Molding, Part 1 · M. Holland: Monitoring Pressure Loss · BASF: Injection-Molding Problems · NIST: Measures of Scale · NIST: Coefficient of Variation · PMC: Filling Imbalance in Multi-cavity Molds · Hingtung: Cavity Balance Study
